A pilot flame is typically used in gas or other fuel systems as a source to ignite the system when a fuel supply is provided in proximity to the pilot flame. Well known pilot flame uses in domestic applications include gas stoves, water heaters, fireplaces and other home heating systems. In the event that a pilot flame is extinguished, the homeowner simply re-ignites the pilot flame. In commercial applications, a pilot flame failure can be much more problematic than the inconveniences occurring in domestic situations. For example, in refineries and oil fields, it is frequently necessary to burn off substantial quantities of waste and perhaps toxic hydrocarbon gases. Typically, a relatively tall stack is employed which will be located outdoors and a substantial distance from any control facility. While various schemes have been proposed for igniting a burnoff flare, starting with bows and flaming arrows, most modern facilities use a pilot burner which maintains a flame adjacent the top of the stack. However, even the most carefully designed pilot burner can be extinguished and it is increasingly undesirable to release waste gas when no immediate source of ignition is present.
Various systems have been proposed for monitoring the presence of a pilot flame. Optical systems have been used to detect the pilot flame from the ground but suffer from false readings due to fog interference or detector mistakes due to direct sunlight. Thermocouples are also available but typically have a short useful life due to the heat in the stack. In addition, maintenance and replacement of the electronics or sensing mechanism can be difficult.
Re-lighting of a pilot burner is typically implemented by means of an igniter tube which extends essentially the full height of the stack. To re-start the pilot burner, this igniter tube is filled with a gas/air mixture which is then electrically ignited at the bottom of the stack. A flame front progresses up the igniter tube until it reaches the top of the stack where it can ignite the pilot burner itself. Such systems are known in the art and are not described in detail hereinafter.
It has been found that the burners used for maintaining the pilot flame emit a characteristic acoustic signature with much of the acoustic energy being contained in a relatively narrow, low frequency band, e.g., 250 Hz to 500 Hz. This frequency band will vary, however, according to the application and magnitude of the pilot flame. Further, if an appropriate microphone or sensor system is available, the acoustic signature of the burner can, to some extent, be tuned to match or cooperate with the characteristics of the microphone or sensor. Sensing can be still further complicated by the fact that the sensing mechanism is often outdoors and thus exposed to undesirable environmental conditions. Therefore, the microphone or sensor may be required to operate in a relatively hostile environment.
Thus, what is needed is a capability to detect a pilot flame according to the acoustic characteristics of the pilot flame with a high degree of accuracy while being located in a location away from the pilot flame and easily accessed for service. The pilot flame detection must be highly reliable even in a hostile environment.